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Abstract:

A method for measuring and controlling a chip level and/or a chip-liquor
combination level and/or liquor level height of a liquid-steam phase
digester or a hydraulic digester which digester comprises a chip screw
for feeding chips into a digesting volume. The method comprises mounting
electrode pairs in certain heights and vertically at distances from each
other in the digesting volume of the digester, the electrode pairs being
formed of electrodes; connecting the electrode pairs to electric
circuits; measuring in each electric circuit a current or voltage value
during operation of the digester; determining, based on the measurement,
type of material layer (for example, steam, chips, combination of chips
and liquor, or liquor) located in a certain height, based on the
electrical conductivity of the materials in the digesting volume. An
arrangement for measuring and controlling a chip level and/or a
chip-liquor combination level and/or a liquor level height of a
liquid-steam phase digester or a hydraulic digester.

Claims:

1. A method for measuring and controlling a chip level and/or a
chip-liquor combination level and/or a liquor level height of a
liquid-steam phase digester or a hydraulic digester which digester
comprises a chip screw for feeding chips into a digesting volume; and the
method comprising mounting electrode pairs in certain heights and
vertically at distances from each other in the digesting volume of the
digester, the electrode pairs being formed of electrodes; connecting the
electrode pairs to electric circuits; measuring in each electric circuit
a current or voltage value during operation of the digester; determining,
based on the measurement, type of material layer located in a certain
height, based on the electrical conductivity of the materials in the
digesting volume.

2. The method according to claim 1 comprising arranging a potential
difference between the electrodes of the electrode pairs and measuring
the current in the electric circuit.

3. The method according to claim 1 comprising supplying a constant
current to the electric circuit of the electrode pairs and measuring the
voltage in the electric circuit.

4. The method according to claim 1 comprising using the determined height
information of the chip level and/or the chip-liquor combination level
and/or the liquor level as control for the digesting process and
repeating the measuring and control steps of the method.

5. The method according to claim 1 comprising arranging a changing
potential difference between the electrodes of the electrode pairs which
is preventing generation of precipitation on electrode surfaces.

6. An arrangement for measuring and controlling a chip level and/or a
chip-liquor combination level and/or a liquor level height of a
liquid-steam phase digester or a hydraulic digester which digester
comprises a chip screw for feeding chips into a digesting volume; and the
arrangement comprises electrode pairs which are formed of electrodes and
mounted in certain heights and vertically at distances from each other in
the digesting volume of the digester; and the electrode pairs are
connected to electric circuits such that, during operation of the
digester, a current or voltage value measured in each electric circuit is
determining the type of material layer located in a certain height, based
on the electrical conductivity of the materials in the digesting volume.

7. The arrangement according to claim 6, wherein the arrangement
comprises a circuit measuring means which is during operation of the
digester arranged to measure current of the circuits when potential
differences are arranged between the electrode pairs.

8. The arrangement according to claim 6, wherein the arrangement
comprises a circuit measuring means which is during operation of the
digester arranged to measure voltage of the circuits when current is
supplied in the circuits.

9. The arrangement according to claim 6, wherein the arrangement
comprises an output for communicating the height information of the chip
level and/or the chip-liquor combination level and/or the liquor level
from the circuit measuring means for the control of the digesting
process.

10. The arrangement according to claim 6, wherein the electrode pairs are
arranged under the chip screw.

11. The arrangement according to claim 6, wherein the arrangement
comprises one or more measuring rods to which is/are mounted electrodes
for the electrode pairs.

Description:

TECHNICAL FIELD

[0001] The invention relates to a method and an arrangement for measuring
and controlling a chip and/or liquor level. The invention relates
particularly, though not exclusively, to measuring of the surface level
of chips in a steam-liquid phase digester and a hydraulic digester.

BACKGROUND ART

[0002] In a continuous pulp digester the chips are fed as a regular flow
to the digester together with the liquor while a corresponding amount of
pulp and liquor is blown out of the digester. There are two types of
continuous digesters, the liquid-steam phase digester and the hydraulic
digester.

[0003] In the liquid-steam phase digester the liquid surface is lower than
the chip surface wherein a steam phase is formed in an upper portion of
the digester. A steam supply is provided via a top of the digester. The
chips can be fed from outside the digesting volume, in the level of the
steam phase, to a chip screw which is feeding the chips upwards to the
top of the digester. The chips are dropping outside the chip screw from
the top of the digester to the digesting volume.

[0004] The hydraulic digester is filled with a digesting liquid, i.e.,
under hydraulic pressure. The chips are brought to the upper portion of
the digester by means of filling liquor and the upper screw in a
cylindrical screen presses the chips into the digesting liquid to the
actual digesting volume. Then the chips proceed downwards in an
impregnation zone and after that reach a cooking zone. In the cooking
zone the temperature is increased to a cooking temperature.

[0005] The surface height of the chips is measured in the liquid-steam
phase digester and the hydraulic digester in order that the cooking
process can proceed undisturbed. The chip surface of the digester can be
influenced by blow flow, bottom scraper rotation speed and chip screw
rotation speed which is regulating the chip feed. The blow flow is
affecting the mass flow which is exiting the digester and the rotation
speed of the bottom scraper is affecting the consistency of the mass in
the bottom of the digester. The described control quantities can be
applied as different combinations.

[0006] The liquor surface can rise above the chip surface in the
liquid-steam phase digester what is causing an abnormal situation. The
situation may arise for example due to disruption in the feed of the
chips what may be caused by a blockage of the input path of the chips.

[0007] In the regulation of the chip surface the surface height is in the
prior art measured with mechanical or, in the liquid-steam phase
digester, with radiometrical sensors. A precipitation which is accruing
onto walls of the steam phase, and scum, are disturbing the radiometrical
measurement. The mechanical surface height sensors are blades which are
mounted inside the digester and measuring torque. The chips on the blade
are causing to the blade a torque which is increasing when the load
against the blade is increasing. There are many measuring variations in
the blade measurements because the height of the chip pile is differing
in different locations of the digester. Further the precipitation is
accumulating on the blades which is increasing measuring uncertainty in
the course of time.

[0008] A mechanical chip surface meter is used in the prior art as a
prolongation of the upper screw of the hydraulic digester wherein torque
is measured by members which are lead through the upper screw and sunken
into the mass. Precipitation is accruing to the mechanical parts wherein
the measuring uncertainty is increasing.

[0009] An object of the invention is to provide an alternative method and
arrangement for measuring the surface height of the chips. An object of
the invention is to eliminate or at least minimize drawbacks in the prior
art. Still an object of the invention is to increase reliability of
surface height measuring of the chips, improve surface height regulation
of the chips and decrease service need of surface height measuring
equipment.

SUMMARY

[0010] According to a first aspect of the invention there is provided a
method for measuring and controlling a chip level and/or a liquor level
height of a liquid-steam phase digester or a hydraulic digester which
digester comprises a chip screw for feeding chips into a digesting
volume, and the method comprising mounting electrode pairs in certain
heights and vertically at distances from each other in the digesting
volume of the digester, the electrode pairs being formed of electrodes,
connecting the electrode pairs to electric circuits, measuring in each
electric circuit a current or voltage value during operation of the
digester, determining, based on the measurement, type of material layer
(for example, steam, chips, combination of chips and liquor, or liquor)
located in a certain height, based on the electrical conductivity of the
materials in the digesting volume.

[0011] Preferably the method comprises arranging a potential difference
(AC, DC) between the electrodes of the electrode pairs and measuring the
current in the electric circuit.

[0012] Preferably the method comprises supplying current to the electric
circuit of the electrode pairs and measuring the voltage in the electric
circuit.

[0013] Preferably the method comprises using the determined height
information of the chip level and/or the liquor level as control for the
digesting process and repeating the measuring and control steps of the
method.

[0014] Preferably the method comprises arranging a changing potential
difference (AC, DC) between the electrodes of the electrode pairs which
is preventing generation of precipitation on electrode surfaces.

[0015] According to a second aspect of the invention there is provided an
arrangement for measuring and controlling a chip level and/or a liquor
level height of a liquid-steam phase digester or a hydraulic digester
which digester comprises a chip screw for feeding chips into a digesting
volume, and the arrangement comprises electrode pairs which are formed of
electrodes and mounted in certain heights and vertically at distances
from each other in the digesting volume of the digester, and the
electrode pairs are connected to electric circuits such that, during
operation of the digester, a current or voltage value measured in each
electric circuit is determining the type of material layer (for example,
steam, chips, combination of chips and liquor, or liquor) located in a
certain height, based on the electrical conductivity of the materials in
the digesting volume.

[0016] When the electric circuits are in an electrified state, the
locations of the vertical chip and liquor zones can be determined based
on the electrical conductivities of the chips, the liquor and the
combination of chips and liquor which are at least approximately known.

[0017] Preferably the arrangement comprises a circuit measuring means
which is during operation of the digester arranged to measure current of
the circuits when potential differences (AC, DC) are arranged between the
electrode pairs. Preferably the arrangement comprises voltage supplying
means for generating potential difference (AC, DC) in the circuits of the
electrode pairs. Preferably the circuit measuring means comprises voltage
supplying means for generating potential difference (AC, DC) in the
circuits of the electrode pairs.

[0018] Preferably the arrangement comprises a circuit measuring means
which is during operation of the digester arranged to measure voltage of
the circuits when current is supplied in the circuits. Preferably the
circuit measuring means comprises current measuring means for measuring
current of the circuits.

[0019] Preferably the arrangement comprises an output for communicating
the height information of the chip level and/or the liquor level from the
circuit measuring means for the control of the digesting process.

[0020] Preferably the electrode pairs, more preferably measuring rods, are
arranged under the chip screw. Preferably the electrode pairs, more
preferably the measuring rods, are arranged under a chip basket.

[0021] Preferably the arrangement comprises one or more measuring rods to
which is/are mounted electrodes for the electrode pairs. Preferably the
measuring rod/rods is/are mounted under the chip screw.

[0022] Preferably the electrode pair is formed in the same measuring rod
of electrodes which are mounted in two different heights.

[0023] Preferably the electrode pair is formed in two or more measuring
rods of electrodes which are mounted in the same height or different
heights.

[0024] Preferably the electrode pair is formed in the same measuring rod
of electrodes which are mounted in the same height and are at a
horizontal distance from each other, preferably at opposite sides of the
rod.

[0025] Preferably the vertical distances of the electrode pairs form the
neighboring electrodes are in the upper end of the measuring rod (in the
gas phase) larger than in the bottom end of the measuring rod.

[0026] With the method and the arrangement the chip surface height
measured in the liquid-steam phase digester or the hydraulic digester can
be exploited in improving the fluency of the cooking process. The
measured chip surface and/or liquor surface height can be used as a
control quantity when controlling the blow flow; the bottom scraper
rotation speed; the chip screw rotation speed and/or material flows of
the digester such as supply of liquor to the digesting volume.

[0027] By the method and the arrangement the level height measurement of
masses in the digesting volume of the continuous pulp digester can be
improved, the reliability of the surface height measurement can be
increased and the reliable lifetime of the measuring means in the
digesting volume can be improved, the surface height regulation of the
masses in the digesting volume can be improved and the service need of
surface height measuring means can be reduced. By the method and the
arrangement the problem of the so called floating chips can be recognized
which can exist both in the liquid-steam phase digester and in the
hydraulic digester. Then the chips don't sink under the liquor but are
floating on the liquor surface. A region is forming under a thin chip bed
where exist sole liquor what can also be detected in the surface height
measurement. Generally the chips are then too dry and the chips can block
quickly the chip feeding openings until the chip feeding screw and the
process is stopping. This failure can be detected proactively by the
surface height measurement and there may be time left for making some
improvements in order to prevent the blockage.

[0028] Different embodiments of the present invention will be illustrated
or have been illustrated only in connection with some aspects of the
invention. A skilled person appreciates that any embodiment of an aspect
of the invention may apply to the same aspect of the invention and other
aspects alone or in combination with other embodiments as well.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention will be described, by way of example, with reference
to the accompanying drawings, in which:

[0030]FIG. 1 shows the method and the arrangement in connection with a
liquid-steam phase digester;

[0031]FIG. 2 shows the method and the arrangement in connection with a
hydraulic digester;

[0032]FIG. 3 shows a first alternative placement of the measuring
electrodes;

[0033]FIG. 4 shows a second alternative placement of the measuring
electrodes;

[0034]FIG. 5 shows the method and the arrangement in an abnormal
situation;

[0037] In the following description, like numbers denote like elements. It
should be appreciated that the illustrated drawings are not entirely in
scale, and that the drawings mainly serve the purpose of illustrating
embodiments of the invention.

[0038] FIGS. 1 and 3 to 5 show a liquid-steam phase digester 100 which
comprises a chip screw 102 in an upper portion 101 of the digester. Feed
of the chips into the digester 100 is illustrated with dashed line
arrows. The chips are fed from outside a digesting volume through an
input channel 103, in the height of a steam phase, to a chip screw 102
which is feeding the chips upwards to a top 104 of the digester. A motor
105 which is located in the top 104 of the digester is rotating the chip
screw 102 which is located in a screen basket 106. During lifting motion
of the chip screw the liquid which is separated from the chips is lead
through the screen basket 106 to a collecting channel 107 and further
outside the digester (arrow 108 in FIG. 1). The chips are dropping
outside the chip screw 102 from the top 104 of the digester to the
digesting volume.

[0039] The steam phase is extending around the chip screw 102 and to a
steam volume 109 below the chip screw. An upper surface of a chip layer
111 is called chip surface 110. The boundary surface between the chip
layer 111 and a combined chip and liquor layer 113 is called liquor
surface 112.

[0040] The arrangement comprises electrode pairs which are formed of
electrodes and mounted at certain heights and vertically at distances
from each other in the digesting volume of the digester 100, and the
electrode pairs are connected to electric circuits 300 such that, during
operation of the digester, a current or voltage value measured in the
electric circuit is determining the type of material layer (for example,
steam, chips, combination of chips and liquor, or liquor) located at
certain height, based on electrical conductivity of the materials in the
digesting volume. The electric circuits 300 are measured by a circuit
measuring means 301 to which preferably the electric circuits of all
electrode pairs are connected.

[0041] Additionally the arrangement comprises an output 302 for
communicating the height information of the chip level 110 and/or the
combined chip and liquor level 113 and/or the liquor level 115 from the
circuit measuring means 301 to be exploited in the control of the
digesting process such as when controlling the blow flow; regulating the
bottom scraper rotation speed; regulating the chip screw rotation speed
and/or regulating the material flows of the digester such as supply of
the liquor into the digesting volume.

[0042] In the method and the arrangement a voltage (preferably a constant
voltage) is regulated between the electrodes of the electrode pairs
forming the measuring circuit which electrodes are mounted in the
digesting volume and the current in the measuring circuit is measured.
Alternatively a constant current can be supplied to the measuring circuit
and the voltage can be measured. The voltage is preferably constant
voltage DC but also pulsed DC or alternating voltage AC can come into
question wherein accruing of precipitation on the electrodes can be
decreased. In the method and the arrangement the chip surface and the
liquor surface height can be measured by one or more measuring rods 2, 3,
4 based on electrochemical measurement and mounted in the digester 100.

[0043] In FIGS. 1 and 6 the measuring rod 2 comprises electrodes E of
which two electrodes are connected in the measuring circuit and placed in
the same measuring rod in different heights, for example, both electrodes
E4-E5 in the steam volume 109; and E5-E6 at both sides of the chip
surface 110. Electric properties of a gas or a mass or a liquid existing
in each height of the digesting volume can be determined with the voltage
or current which is connected to the electrodes E. The surface height of
the chips or liquor can be deduced from the measured properties such as
electric conductivity in a normal operation situation and an abnormal
situation wherein for example liquor can be accrued to a layer above the
chips.

[0044] Electrodes E are preferably arranged in an upper part 2' of the
measuring rod 2 in a region of the steam phase of the digesting volume
with larger distances than in a lower part 2'' of the measuring rod in an
actual location region of the mass and the liquid in a height of the chip
layer 111 and the combined chip and liquor layer 113 (liquid phase). The
electrodes which are located more sparsely, in a height of an unlikely
location height of the mass, are saving costs of the construction. The
electrodes E placed in different heights in the measuring rod 2 are
denoted with an indexing E(1+n), where n=1, which is continuing downwards
from top. For the sake of clarity only a part of the electrodes are named
in the figures.

[0045] The distance between the electrodes is preferably 30 to 40 cm in
the upper end 2' of the measuring rod located in the steam volume
(particularly in the upper portion of the steam volume), and lower than
that the distance is smaller. Naturally, the distance between the
electrodes must not be constant in any region of the measuring rod when
the location of the electrodes is known in the vertical direction of the
digesting volume, in order to control the cooking process based on the
measuring results of the surface heights.

[0046]FIG. 2 shows the method and the arrangement in connection with the
hydraulic digester 200. Feed of the chips into the digester 200 is
illustrated with dashed line arrows. The chips are fed through an input
channel 203 to a chip screw 202 which is located in an upper portion 201
of the digester 200. The chip screw 202 is feeding the chips downwards
from a top 204 of the digester. A motor 205 which is located in the top
204 of the digester is rotating the chip screw 202 which is located in a
screen basket 206. During rotating motion of the chip screw the liquid
which is separated from the chips is lead through the screen basket 206
to a collecting channel 207 and further outside the digester (arrow 208
in FIG. 2). The chips are dropping under the chip screw 202 to an upper
portion 209 the digesting volume which is filled with liquor. An upper
surface of a chip layer 111 is called chip surface 110. In the hydraulic
digester there can be found only the upper portion filled with liquor and
a lower portion which is covered with the combined chip and liquor layer.
The chip surface must not rise too high wherein the digester starts to
become blocked. On the other side the chip surface must not drop too low
wherein the capacity of the cooking process collapses and the quality of
the fiber suffers from the excessive alkali dosage.

[0047] A space 220 through the rotation motor 205 and the chip screw 202
is arranged in the digester 200 for placing the measuring rod 2 in the
digesting volume on the rotation axis 202' of the chip screw. The space
can exist for the known mechanical surface height measuring apparatus
wherein the digester is easy to rebuild according to the arrangement of
the invention.

[0048] In connection with FIG. 2 it is referred to FIG. 1 where a
corresponding measuring rod 2, electrodes E, electric circuits 300,
measuring means and output 302 are described. The digesting volume 209 in
FIG. 2 is deviating from the digesting volume in FIG. 1 below the chip
screw 202 in that the digesting volume 209 is filled with liquid.

[0049] The electrical conductivity in different layers of the digesting
volume changes depending on the conductivity in the material which is
located between the electrodes of the electrode pair. A closed electric
circuit is substantially not generated in the steam volume 109 (if at
least one electrode is in the steam volume), i.e., there flows no
current. There flows current in the chip layer 111 depending on the
amount of liquor in the chips when a potential difference is generated in
the electric circuit. In an abnormal situation (FIG. 5) the amount of
current in the liquor layer 115 above the chip layer 111 is larger
related to the combined chip and liquor layer 113 because the undiluted
fresh liquor is conducting electricity better than the combined layer.

[0050] The potential difference to be connected between the electrode
pairs can be about 2V wherein the current in the electric circuit can be
more than 100 mA, maximally about 2 A. The current is preferably at a mA
level when current is supplied to the electric circuit.

[0051] The examples of arranging one measuring rod and electrode pairs
described in connection with FIGS. 1, 3, 5, 6 and 7 can be applied also
with the hydraulic digester 200 in FIG. 2, preferably when the measuring
rod is placed on the rotation axis of the feeding screw. Examples of
arranging electrodes to pairs are described in the following.

[0052] In FIG. 1, the electrodes E1 . . . E5 are located in the steam
volume 109, the electrodes E6 . . . E11 in the chip layer 111 below the
steam volume and the electrodes E12 . . . E14 are located in the liquor
layer 113 below the chip layer 111. The electrodes are connected in
electric circuits such that there are formed electrode pairs.

[0053] In FIG. 2, the electrodes E1 . . . E14 are arranged to electrode
pairs corresponding to FIG. 1, for example, E1-E2, in certain heights
vertically at distances from each other such that the electrodes of the
electrode pair connected in one electric circuit 300 are placed in
different heights in the same measuring rod.

[0054]FIG. 3 shows an example of placing electrodes in two or more
measuring rods 2, 3. Two electrodes A, B which are connected in a
measuring circuit are placed in separate measuring rods 2, 3 in the same
height, for example, the electrodes A2-B2 in the steam volume 109; and
A9-B9 in the chip layer 111. Naturally, electrodes located in two or more
measuring rods and connected to the same measuring circuit can locate in
different heights.

[0055] In FIG. 3, the electrodes placed in neighboring measuring rods 2
and 3 are denoted in the same height in the following way: in the first
measuring rod 2 with an indexing A(1+n), where n=1 which is continuing
from above downwards, and in the second measuring rod 3 with an indexing
B(1+n), where n=1 which is continuing from above downwards.

[0056] In FIG. 3, the electrode pairs A1-B1 . . . A5-B5 are located in the
steam volume 109, the electrode pair A6-B6 in the height of the chip
surface 110, the electrode pairs A7-B7 . . . A11-B11 in the chip layer
111 and the electrode pairs A12-B12 . . . A14-B14 in the chip-liquor
layer 113 below the chip layer 111.

[0057] FIGS. 4 and 7 show two electrodes which are connected in the same
measuring circuit and placed in the same height in the same measuring
rod, for example, both electrodes C1-D1 in the steam volume 109.

[0058] In FIG. 4, the neighboring electrodes placed in the same measuring
rod 2 at opposite sides are denoted in the same height with an indexing
C(1+n) and D(1+n) where n=1 which is continuing from above downwards.

[0059] In FIG. 4, the electrode pairs C1-D1 . . . C5-D5 are located in the
steam volume 109, the electrode pair C6-D6 in the height of the chip
surface 110, the electrode pairs C7-D7 . . . C11-D11 in the chip layer
111 and the electrode pairs C12-D12 . . . C14-D14 in the chip-liquor
layer 113 below the chip layer 111.

[0060]FIG. 5 shows the method and the arrangement in an abnormal
operation situation of the liquid-steam phase digester 100.

[0061] The surface of the liquor in the digester is risen above the chip
surface wherein the liquor layer is above the chip layer when the chip
layer 111 is sunken abnormally deep inside the liquor. The situation may
arise for example due to disruption in the feed of the chips what may be
caused by a blockage of the input path of the chips.

[0063] Examples of arrangements in which a constant voltage is regulated
in electric circuits which are connected between an electrode pair, and
the current in the electric circuits is measured is shown in Tables 1 to
4. The voltage can be connected to all measuring circuits at the same
time. It is preferable to measure the current of the electrode pairs in
the arrangement alternately in suitable time intervals (to poll
alternately the electric circuits of the electrode pairs) for example in
intervals of some seconds. In the measurement can be determined a) if the
current flows or not, and if desired, additionally b) the amount of the
current.

[0064] Table 1 shows the relations of the voltage and the current when the
digester 100 is in the normal state which is shown in FIG. 1.

[0068]FIG. 6 shows a side view and a top view of a portion of the
measuring rod 2 in more detail. The measuring rod 2 comprises a body 10,
an isolation layer 11 which is surrounding the body and an electrode E1
outside the isolation layer. Preferably the body is metal, for example,
stainless steel which is suitable in the circumstances of the digester.
The material of the isolation layer 11 can be isolating material
polytetrafluoroethylene (PTFE, Teflon) which is suitable in the
circumstances of the digester without softening too much. FIG. 6 shows a
circular single electrode E1 which is preferably of stainless steel and
fixed to the body by a screw 12. An electric wire is leading from the
electrode to the electric circuit 300 inside the body 10.

[0069]FIG. 7 shows a side view and a top view of a portion of the
measuring rod 4 in more detail. The measuring rod 4 comprises the body 10
and the isolation layer 11 corresponding to FIG. 6. Additionally the
measuring rod comprises in the same height two electrodes C1 and D1 which
are connected to the electric circuit 300 by wires which are lead inside
the body. The electrodes shown in FIG. 7 are designed rounded in their
shapes such that they resist flow marginally what is reducing the
precipitation. If desired, the electrodes comprise above and below
chamfers 13.

[0070] The foregoing description provides non-limiting examples of some
embodiments of the invention. It is clear to a person skilled in the art
that the invention is not restricted to details presented, but that the
invention can be implemented in other equivalent means. Some of the
features of the above-disclosed embodiments may be used to advantage
without the use of other features.

[0071] As such, the foregoing description shall be considered as merely
illustrative of the principles of the invention, and not in limitation
thereof. Hence, the scope of the invention is only restricted by the
appended patent claims.

Patent applications by Martti Pulliainen, Mikkeli FI

Patent applications in class With testing, sampling or analyzing

Patent applications in all subclasses With testing, sampling or analyzing